Microsatellites are DNA stretches in the genome composed of repeats of short nucleotide sequences one to six nucleotides repeated n times. There are thousands of these in the genome and they are individually defined by their unique flanking regions and therefore amplifiable (by PCR) and so visualized. They are polymorphic (as number of repeats at an allele) and generally stably inherited, although shown to mutate in number of sequence repeats at an allele) and generally stably inherited, alth0ugh sown to mutate in number of sequence repeats at appreciable frequencies (0.01-0.0001). In certain types of tumors they have been shown to be highly unstable due to deficiencies in mismatch repair genes. Deficiencies at other loci may also enable such instability. Quantitative measures of specific microsatellite locus instability (MSI) have not been developed. We have developed a procedure, small pool PCR (SP-PCR) capable of carrying out that quantitation and coupled that with a high throughput fluorescent method to screen for MSI at frequencies at minisatellite loci. We will direct these techniques to families inheriting multiple cancer predisposition Li/Fraumeni Syndrome (LFS) and Wilms tumor syndrome (WT) and answer a series of questions. What is the level of instability in LFS tumors of different types and WT tumors? Is instability different in tumors with p53 mutation as opposed to tumors in families where p53 mutation have not been identified, and do those differences reflect different loci responsible for instability? The same question will be asked about WT tumors in families were the candidate gene is linked to 19q versus those where it is not. When is instability manifested in cultured fibroblasts from LFS patients as cells pass from pre-senescence to immortalization to tumor forming? Is instability detectable and quantitatively different in the "normal" surrounding tissue and peripheral blood lymphocytes of patients inheriting the gene responsible for cancer versus their unaffected sibs? Can those differences be used for presymptomatic diagnosis in families where the mutation responsible for the disorder is not known?